HID: wacom: Delete an unnecessary check before kobject_put()

The kobject_put() function tests whether its argument is NULL and then
returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>